This invention relates to an intraocular lens (IOL) and more particularly to an IOL with a deformable optic which enables the IOL to be passed through an incision, for example, a scleral tunnel incision having an end-to-end dimension no larger than about 3.2 mm or no larger than about 2.8 mm, into the eye. This end-to-end dimension of the incision, that is the shortest distance along the surface being cut from one end of the incision to the other end of the incision, is hereinafter referred to as the dimension of the incision.
When the natural lens of the human eye becomes impaired through, for example, cataracts or injury, it is common practice to replace the natural lens with an IOL. One way to accomplish this is to form a relatively long incision in the eye and remove the natural lens in one piece. However, presently a much more common way to accomplish this is to form a shorter incision in the eye and insert a probe or a phaco tip of a phacoemulsification instrument through the incision into the eye to break up the natural lens using ultrasonic energy. The lens fragments are then aspirated from the eye through the relatively short phaco incision and the phaco tip is removed.
The dimension of the incision, which is commonly referred to as a phaco incision, in the eye through which the phaco tip is inserted is typically no larger than about 2.8 mm or about 3.2 mm so that the phaco tip will fit somewhat snugly through the incision. It is generally recognized that the larger the lens-removal incision, the greater the trauma to the eye and the longer the patient recovery time. Other complications may also be caused by a large incision such as the need to remove the lens in one piece. About 70 percent of the lens-removal processes currently use phacoemulsification techniques. Of course, with only a small incision being needed for phacoemulsification lens extraction, it is desirable that IOL""s be available and used that can be inserted through the small phaco incisions.
A typical IOL includes an optic, usually having a diameter of about 6 mm, and fixation members coupled to (or formed with) the optic to fix the optic within the eye in the region of the extracted lens. IOL""s are of two basic types, those having a hard or rigid optic formed, for example, of polymethylmethacrylate (PMMA) and those having a deformable optic which is constructed of a deformable material such as silicone, hydrogel, or an acrylic. If a hard IOL is used, the small phaco incision must be enlarged to approximately the diameter of the hard optic, in order to permit the hard optic to be inserted through the incision and much of the advantage of phacoemulsification lens extraction is thereby obviously lost.
There are two known classes of prior art IOL""s which can be deformed (e.g., folded or rolled) to pass through a scleral tunnel incision having a dimension of about 3.2 mm. A scleral tunnel incision is a single incision in the sclera without any additional incision which crosses or intersects the single incision. The first of these IOL""s has an acrylic optic with a refractive index of about 1.47 or greater. The acrylic IOL""s cover a full diopter range of about 12d to about 24d, but do not have as high an elongation as is desired for IOL insertion through a small phaco incision. Elongation is defined as (Ld/Lu) times 100 where Ld is the maximum change in length from the unstressed condition to the breaking point and Lu is the unstressed length. A high elongation is desired so that the optic can be caused to resiliently stretch and flow to assume a small cross sectional configuration for passage through a small phaco incision. For example, a currently known acrylic optic may have an elongation of only about 150 percent.
The second of these classes of IOL""s has a silicone based optic. In one known silicone based optic the refractive index is only about 1.408. Accordingly, it is necessary to have a relatively large maximum cross sectional area of about 5.1 square millimeters in order to provide an IOL of only 12 diopter power. Although higher diopter powers can be constructed with this silicone based material, the higher powers require a correspondingly greater maximum cross sectional area with the result that they will not ordinarily pass through a scleral tunnel incision having a dimension of only about 2.8 or about 3.2 mm in the eye. More specifically, the present inventors understand that it is highly unlikely that this type of silicone IOL is implantable, using a stainless steel folding forceps-type inserter, through a 3.2 mm scleral tunnel incision in powers over 14d and that in powers over 15d implantation through a 3.2 mm scleral tunnel incision into the eye is essentially not possible. Implantation of an IOL of this type having a power of 20d or greater is not possible.
A second kind of known silicone based optic is disclosed in Fedorov et al U.S. Pat. No. 4,647,282. One of the silicone based materials disclosed in this patent is said to have a refractive index of 1.480. However, this silicone based material has a percentage of elongation of only 130 percent, and the patent lacks, among other things, specific geometrical teachings as to how the optic is to be constructed.
A third kind of known silicone based optic is disclosed in application Ser. No. 562,452 filed on Aug. 1, 1990 and entitled OPTICALLY CLEAR REINFORCED SILICONE ELASTOMERS OF HIGH OPTICAL REFRACTIVE INDEX AND IMPROVED MECHANICAL PROPERTIES FOR USE IN INTRAOCULAR LENSES. At present, a known 16 diopter power optic of this material has a maximum cross sectional area of about 5.3 sq. mm.
Another important consideration in producing small deformable IOL""s is the placement of the fixation members or haptics, used to assist in retaining the optic in the eye, relative to the optic. Because of strength, molding and other material concerns, it has heretofore been considered necessary to secure the proximal end of the fixation members in the optical zone of the optic of such IOL""s, for example, when the IOL is constructed so as to be deformed and placed in the eye through a small incision. This construction has at least one disadvantage, a portion of each of the fixation members exists in the optical field defined by the optic and, therefore, can interfere with the patient""s vision. This problem is exacerbated by the current trend to smaller and smaller IOL optics. It would be advantageous to provide an IOL including one or more fixation members, for example, of the filament type, in which the optical zone of the optic is completely free of the fixation members.
New IOL""s have been discovered. In one embodiment, the IOL""s of the present invention include optics with optical zones having a periphery with a reduced thickness (parallel to the optical axis of the optic) relative to prior art lenses of the same type. This reduced thickness allows the optic to be deformed, for example, rolled or folded, for insertion in the eye through a very small incision, preferably having a dimension on the order of about 3.2 or even about 2.8 mm. Such reduced thickness is achieved without any substantial detriment, for example, to the optical properties and power of the optical zone of the optic. Consequently, the phaco incision, which is commonly used for the phaco tip, need not be enlarged in order to permit the IOL to be implanted. The fixation member or members of the present IOL""s are preferably secured to the peripheral zone circumscribing the optic so as not to interfere with the optic. In short, the present invention provides very effective and useful small incision IOL""s the optics of which have optical zones with reduced peripheral thickness and/or are free of fixation member interference.
When an IOL of this invention is implanted in the eye, the optic has sufficient rigidity to be substantially free of optical distortion resulting from force from the eye acting on the IOL. The optic is preferably of sufficient size and/or is suitably structured to reduce, or even substantially prevent, the glare that would result if the dimensions of the optic were so small that light could interact with the periphery of the optic to cause glare. In one embodiment, the optical zone of the optic is constructed of optically clear material, and the peripheral zone, which circumscribes the optical zone, is structured to provide reduced glare relative to a substantially identical IOL with the peripheral zone made from the same optically clear material. For example, the peripheral zone of the optic can be at least partially roughened. Such roughening acts to reduce the glare exhibited by the implanted optic relative to a substantially identical optic without a roughened peripheral zone.
The optic preferably has an elongation of at least about 160 percent or about 200 percent. Preferably, the elongation of the optic is at least about 260 percent.
This invention is applicable to a very high percentage of the diopter powers currently being employed in IOL""s. The invention is specifically applicable to deformable optics with optical zones having from about 12 to about 24 diopter power, and this range of diopter powers is believed to be prescribed for about 95 percent of all IOL recipients. The features of this invention are applicable, for example, to an optic with optical zones having at least about 20 diopter power, and this is a higher power than the diopter power of known prior art silicone IOL""s that were implantable through a 3.2 mm scleral tunnel incision.
This invention is based, in part, upon the recognition by the present inventors that the maximum cross sectional area of the optic is a primary controlling factor in determining the dimension of the incision required for insertion of the deformable optic through the incision. More specifically, for insertion through scleral tunnel incisions having a dimension of no greater than about 3.2 mm or about 3.0 mm, the maximum cross sectional area of the optic should be no greater than about 5.8 square mm.
In addition, the present inventors have discovered that the peripheral thickness of the optical zone can be reduced without substantially detrimentally effecting the optical properties of the optical zone. More specifically, the cross-sectional area of the optic can be reduced (and maintain the same optical power) by forming the optic so that the peripheral zone has a maximum axial thickness (parallel to the optical axis) which is larger than the maximum axial thickness of the periphery (which may be imaginary) of the optical zone. Of course, in accordance with the present invention, the optical power of the optical zone can be increased while maintaining the cross-sectional area of the optic constant. The reduction in optic cross-sectional area or the increase in optical power is relative to a substantially identical IOL in which the peripheral zone has a maximum axial thickness which is the same as the maximum axial thickness of the periphery of the optical zone. In the present invention, the peripheral zone is maintained at a sufficient thickness so as to allow the fixation member or members of the IOL to be firmly secured to the optic.
The optic of this invention has an optical axis, anterior and posterior faces, and the curvature of one or both of these faces determines the corrective or diopter power of the optic. In order to keep the maximum cross sectional area at or below the desired maximum, it is desirable to minimize the convex curvature of the face or faces which provide the correction. To obtain the necessary correction without undue convex curvature which would thicken the optic, it is preferred, but not essential, to employ an optic having an index of refraction of at least about 1.40 with an index of refraction of at least about 1.46 being preferred. An index of refraction less than 1.40 tends to limit the diopter correction that is obtainable or require other trade offs that may be undesirable.
It is also important that the optic not be made so thin that, when implanted and subjected to the usual forces from the eye, it buckles or deforms and thereby introduces optical distortion. The relatively thick peripheral zone of the present IOL""s inhibits, to some extent, such buckling or deforming. The peripheral zone preferably forms, in effect, a frame which assists in strengthening the optic against unwanted deformation after implantation. The peripheral zone preferably includes buttresses for use in attaching the fixation members to the optic and for lending support to the optic. For one preferred construction, the thickness of the optic along the optical axis is no less than about 0.65 mm or about 0.68 mm to about 0.79 mm in order to guard against the optical distortion that would result from mechanical deformation of the optic of the implanted IOL.
The optic should also be constructed so as to reduce or even substantially prevent glare. To accomplish this, the optic, including the optical zone and peripheral zone, should preferably have sufficient radial dimensions to cover the optical zone within the eye to assure that light rays do not interact with the edges of the optic sufficiently to cause glare. In a preferred construction, the optic is circular and has a diameter of at least about 6 mm, although glare can be substantially prevented by an optic having smaller diameters down to about 5 mm. All or a portion of the peripheral zone of the optic may be structured, for example, have at least a portion of its surface roughened, to provide reduced glare relative to a substantially identical intraocular lens with the peripheral zone made from an optically clear material.
Fixation members are used to fix the optic within the eye. Another important consideration is constructing the optic in a way that enables the obtaining of a strong attachment between the fixation members and the optic. The peripheral zone is not used for focusing of light on the retina, but is used for receiving attachment regions of the fixation members for attaching the fixation members to the optic. The central optical zone is used for focusing light on the retina and providing the desired correction. The peripheral zone and fixation member or members are preferably constructed so that the optical zone of the optic is completely free of the fixation member or members. In a particularly useful construction, the proximal end portion of one or more of the fixation members, for example, that portion of the fixation member located in the optic, extends in a direction which is generally tangential to the optic.
Because the peripheral zone adds to the maximum cross sectional area of the optic and does not effectively contribute to the optical properties, for example, the optical power, of the IOL, it is desirable to reduce the axial cross sectional area of the peripheral zone to a minimum. On the other hand, a certain thickness, i.e. axial dimension, of the peripheral zone is needed in order to form a strong attachment with the fixation members and/or to provide advantageous strength to the optic. In a preferred construction, the thickness of the peripheral zone in the axial direction is no less than about 0.305 mm with a thickness no less than about 0.381 mm being more preferred.
Other properties of interest of the optic include hardness and tensile strength. Preferably, the hardness of the optic is at least about 38 Shore A so that compressive forces exerted on the optic by the tool used for implanting the IOL is less likely to permanently scratch or mark the IOL. In addition, the optic preferably has a tensile strength of no greater than about 1000 psi because tensile strengths greater than this make the optic more difficult to elongate during insertion. The tensile strength should preferably be greater than about 600 psi in order to prevent mechanical distortion of the optic when the fixation members are resiliently urged radially inwardly.
The optics of the present IOL""s may be constructed of any suitable material or combination of materials. Such materials include, for example, silicone based materials (polymeric materials), acrylic polymeric materials, hydrogel-forming polymers and the like and mixtures thereof. Certain silicone based materials possess a number of properties which adapt them for use as the optic material. Preferably the silicone based materials are substantially free of acrylates. Preferred silicone based materials are described in application Ser. No. 562,452 filed on Aug. 1, 1990 referred to above and entitled OPTICALLY CLEAR REINFORCED SILICONE ELASTOMERS OF HIGH OPTICAL REFRACTIVE INDEX AND IMPROVED MECHANICAL PROPERTIES FOR USE IN INTRAOCULAR LENSES. This application is incorporated in its entirety by reference herein.
The fixation members can be of various configurations and material. For example, the fixation members may be constructed of PMMA or polypropylene. To more desirably apply the forces on the optic caused by the eye after implantation, each of the fixation members is preferably in the form of a C-shaped elongated resilient member. Preferably the fixation members are coupled to the optic at generally diametrically opposed locations.
The invention, together with additional features and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying illustrative drawings.